As always, I would like to generate some prior art to extend room of operation.  The following concerns the situation that multiple scan heads are used in a single machine. For an example see Kleo which uses 288 bundles by Carl Zeiss.

If multiple scan heads are used, a challenge is to align these heads. The position of the laser in each head has to be known exactly.
One way of doing this is by moving a camera under the scan heads and collecting position information per laser. The laser would project directly onto the CCD / CMOS chip and its position would be determined. This is however expensive as it requires an extra stage with camera. The chip has a finite size of for example 5x5 mm and has to be moved.

Another solution of doing this would be to add a bar from diffuse glass, e.g. opal glass. The light would be scattered in this bar and reach the edges of it. At the edge of this bar there would be a photodiode. The bar could be made of opal glass. One might think that this  bar must be narrow so the position can be detected up to 10 micrometers accurate in one direction. The current photodiode used to calibrate the laser is, however, also not narrow.  You can simply use the rising edge of the signal recorded by the photo-diode used to monitor the diffuse opal bar.
The stage upon which the scan head is mounted then moves in  orthogonal direction to this bar.
By turning on the laser and moving it over the bar. The position can be determined exactly in that direction. It might be needed to add a cap-around the bar to minimize stray light. This is also done for the photo-diode  in the scanhead.
Still, I need two dimensional information. I also need to know the position of the laser diode at the bar.
To do this I could use multiple photodiodes along edges of the bar. They would all measure a signal if the laser hits the bar. The signals will, however, arrive at different points in time. This allows one to determine the position of the laser along the bar.